Ordering Concrete by Volume: Slabs, Footings and Posts

Ordering concrete is unforgiving. You cannot buy half a cubic yard, you cannot return what you do not pour, and a short load means a cold joint that weakens the slab. Most homeowners order 10-20% more than they need and pay for concrete that hardens in the truck. Getting the volume right is simple arithmetic done carefully.

Volume is length × width × thickness, converted from cubic feet to cubic yards by dividing by 27. For anything more than a single square slab, a calculator that sums multiple shapes avoids arithmetic errors — you can calculate cubic volume for slabs, footings and posts before calling the supplier.

Volume by Project Type

Project	Typical Dimensions	Cubic Yards
Patio slab, 4 in thick	12 ft × 12 ft	1.78
Driveway slab, 5 in thick	20 ft × 24 ft	7.41
Shed foundation, 6 in thick	10 ft × 12 ft	2.22
Fence post, 8 in diameter	3 ft deep, 10 posts	0.39
Deck pier, 12 in diameter	4 ft deep, 6 piers	0.84

Walking Through a Driveway

A 20 ft × 24 ft driveway at 5 inches thick: 20 × 24 = 480 sq ft. Five inches is 0.4167 ft, so 480 × 0.4167 = 200 cubic feet. Dividing by 27 gives 7.41 cubic yards. Add the standard 10% waste allowance and you need 8.15 yards. Suppliers round to the nearest quarter yard, so order 8.25.

Note how sensitive the total is to thickness. Drop from 5 to 4 inches and the same driveway needs 5.93 yards — a saving of 1.5 yards, roughly $225 at $150 per yard. Four inches is adequate for passenger vehicles on a properly compacted base. Five to six inches is the right call for anything heavier, and the extra cost is far less than repouring a cracked slab.

Mixing in Bags Instead

For small pours under one cubic yard, bagged mix is practical. An 80-lb bag of concrete yields about 0.6 cubic feet. One cubic yard is 27 cubic feet, so a full yard would need 45 bags — which is why bagged mix stops making sense around half a yard. A single 3 ft × 3 ft × 4 in pad needs 3 cubic feet, or 5 bags.

Frequently Asked Questions

How much waste allowance should I add? 10% for slab work on level ground, 15% for footings dug by hand or any pour over uneven subgrade. The cost of a short load exceeds the cost of the extra concrete.

Does the subgrade thickness count? No. Gravel base is measured and ordered separately. Concrete volume covers only the concrete itself.

What is the minimum practical delivery? Most ready-mix suppliers have a 1-yard minimum and charge a short-load fee below 3 yards. Below one yard, bagged mix is almost always cheaper despite the labor.

Slabs and posts rarely stand alone on a project — footings, gravel and reinforcement all need quantities too. Once you have the concrete figure, size the base material with the same approach and track the overall spend with the site’s home maintenance cost calculator.